How Fit are Floaters? Consequences of Alternative Territorial Behaviors in a Nonmigratory Sparrow

Yearling male song sparrows on Mandarte Island, British Columbia, fell into two social categories: territorial birds and nonterritorial floaters. Some territorial yearlings failed to gain mates. Mated yearlings with territories and floaters were much more common than unmated yearlings with territories. Floaters made up a higher proportion of the population in years of high population density. Unmated territorial birds accounted for a higher proportion of all yearling males at low population densities. Most floaters and unmated territorial yearlings remained unmated during their first breeding season. Floating males were more likely to disappear from the island than territorial males after the first year of life. Nearly all birds that survived to 2 years of age defended territories and bred. Birds that were floaters as yearlings and later gained a territory did not reproduce better in later life than birds that acquired territories as yearlings. As a result of these patterns, males that obtained territories and mates as yearlings raised more than twice as many breeding offspring during their lifetimes as floaters. Yearling territorial males without mates survived well but did not breed more successfully than floaters over their life spans. Defending a territory without a mate and living as a floater were densitydependent alternative reproductive behaviors in yearling male song sparrows. Territoriality, however, was greatly superior to floating as a social and reproductive strategy for yearling males.